This application claims the priority of German patent document 199 22 962.7, filed May 19, 1999, and PCT International Application No. PCT/EP00/02518, filed Mar. 22, 2000, the disclosures of which is expressly incorporated by reference herein.
The invention relates to a method for the periodic desulphurization of a nitrogen-oxide or sulphur-oxide accumulator of an exhaust-gas cleaning system of a combustion machine, such as an internal combustion engine.
Exhaust-gas cleaning systems having a nitrogen-oxide (NOx) accumulator, such as an NOx, adsorber catalytic converter, and optionally a sulphur-oxide (SOx) accumulator, such as a so-called SOx trap, connected upstream of the nitrogen-oxide accumulator, are used in particular in motor vehicles in order to minimize the nitrogen oxide emissions from the motor vehicle internal-combustion engine. For this purpose, it is known, in engine operating phases in which high levels of nitrogen oxide are formed, such as in lean-burn mode, for nitrogen oxide to be temporarily stored in the NOx accumulator, for example by an adsorption process, so that it can be released again and converted in a suitable subsequent operating phase, for example by means of a corresponding desorption process and subsequent reduction to form nitrogen in a rich-burn mode. In this context, the terms lean-burn and rich-burn mode are to be understood as meaning, as is customary, an engine operating phase involving a lean or rich engine air ratio, i.e., an air/fuel ratio of the air/fuel mixture which is burnt in the engine lying above or below the stoichiometric value, respectively.
One known difficulty of such systems is that, particularly in lean-burn mode of the engine, the exhaust gas contains sulphur dioxide on account of sulphur which is contained in standard fuels and engine oils. This sulphur dioxide may lead to sulphur poisoning of the NOx accumulator as a result of sulphate formation, which reduces the NOx storage capacity of the accumulator. Therefore, it is known to subject the NOx accumulator to a desulphurization operation whenever a significant drop in its NOx storage capacity is observed, in order to remove sulphate deposits therefrom. Alternatively, an SOx trap is connected upstream of the NOx accumulator, and is then periodically desulphurized in a similar way.
To effectively desulphurize the nitrogen-oxide or sulphur-oxide accumulator, it is known to set elevated exhaust-gas temperatures (for example, over 600_C) and a rich accumulator air ratio (i.e., a ratio which is below the stoichiometric level). The term accumulator air ratio is understood to mean the ratio of oxygen or air to fuel or unburnt hydrocarbons in the exhaust gas which is fed to the accumulator. Setting such conditions is described, for example, in W. Strehlau et al., New Developments in Lean NOx Catalysts for Gasoline Fueled Passenger Cars in Europe, SAE 96 2047, 1996.
In European patent document EP 0 636 770 A1, it is proposed, in order to desulphurize an NOx adsorber catalytic converter, to switch the internal-combustion engine from a lean engine air ratio to a rich engine air ratio and, if necessary, also to activate an electric heater device associated with the NOx adsorber. The desulphurization phase is maintained each time for a specific period of approximately 10 minutes.
Various methods of the type mentioned in the introduction have also been proposed for carrying out the desulphurization. In these methods, a rich engine mode is combined with a supply of secondary air to the nitrogen-oxide or sulphur-oxide accumulator. In a method of this type which is disclosed in German patent document DE 195 22 165 A1, in addition to other types of procedures, this is achieved by regulating the temperature of the NOx accumulator catalytic converter by suitably setting the engine air ratio and the secondary air quantity in combination with a delayed adjustment of the ignition time to a desired, increased desired value. In a method which is described in the older German patent document 197 47 220.2, the engine air ratio and the secondary air feed rate are set at least as a function of the output signal from a lambda probe arranged downstream of an NOx accumulator catalytic converter which is to be desulphurized, i.e. as a function of the accumulator air ratio. In a further method, which is described in the older German patent document 198 02 631.5, an SOx trap connected upstream of an NOx adsorber catalytic converter is periodically desulphurized with a secondary air supply and a rich engine mode. The SOx accumulator air ratio and the SOx accumulator temperature are held at predetermined desired values, in particular by suitably varying the secondary air quantity supplied and/or the engine air ratio.
One object of the invention is to provide a method of the type described above, which is particularly effective for desulphurizing a nitrogen-oxide or sulphur-oxide accumulator of an exhaust-gas cleaning system of an internal-combustion engine, by secondary air supply and setting a rich engine air ratio.
This and other objects and advantages are achieved by the method according to the invention, in which after a predeterminable desulphurization temperature has been reached, at least during part of the subsequent desulphurization period, the accumulator air ratio (that is, the oxygen/reducing agent ratio) of the exhaust-gas atmosphere in the NOx or SOx, accumulator which is to be desulphurized is set, by suitably changing the secondary air quantity supplied and/or the engine air ratio, (i.e., the air/fuel ratio of the air/fuel mixture which is burnt in the internal-combustion engine), to oscillate between an oxidizing atmosphere and a reducing atmosphere.
This procedure makes use of the discovery that in a reducing atmosphere (i.e., with a rich accumulator air ratio), sulphates present in the nitrogen-oxide or sulphur-oxide accumulator are broken down to form sulfides. The latter cover a catalytically active precious metal material which is usually present in the accumulator body and may lead to a reduced catalytic activity thereof, making further sulphate decomposition more difficult and slower. The inventive insertion of desulphurization phases with at least a stoichiometric, and preferably lean, accumulator air ratio (i.e., a stoichiometric or oxidizing atmosphere), allows the sulfides formed to be broken down so as to release sulphur dioxide (SO2); thereafter, in a subsequent reducing phase of the desulphurization operation, the sulphate decomposition can be continued effectively again. The alternating change in the accumulator air ratio from rich to stoichiometric or lean and vice versa is achieved by suitably changing the quantity of secondary air and therefore oxygen supplied, and/or by changing the engine air ratio and therefore the reducing agent quantity. The oscillating modulation of the accumulator air ratio is preferably maintained until most of the accumulated sulphates have been broken down and desorbed, so that the NOx or SOx storage capacity of the nitrogen-oxide or sulphur-oxide accumulator is once again sufficiently high.
In one embodiment of the invention, the amplitude and/or frequency of the oscillating accumulator air ratio can be set variably according to operating conditions, so that in the corresponding operating situation the desulphurization of the nitrogen-oxide or sulphur-oxide accumulator is as rapid as possible.
In a second embodiment of the invention, the accumulator air ratio is regulated in an oscillating manner between an oxidizing or stoichiometric atmosphere and a reducing atmosphere (i.e., is regulated to a correspondingly oscillating desired value), the actual value being recorded by means of a lambda probe which is positioned suitably upstream or downstream of the nitrogen-oxide or sulphur-oxide accumulator.
In still another embodiment, at the start of the desulphurization process until a predeterminable desulphurization temperature is reached, the accumulator air ratio is kept constantly at least slightly in the rich range by suitably setting a rich engine air ratio despite the secondary air feed having been activated, so that by oxidation of reducing agent the accumulator can be brought to the desired desulphurization temperature relatively quickly.